Stereoisomerism
In the 38th Millennium, the forces of chaos unleashed a new power to the universe. Isomerism, Constitutional and Stereo The term isomers comes from some the names of Alexander The Gr8's trusted companions, isos and meros,whose names mean " Made of the same parts". Since life is hard, this means that Isomers are compounds that are constructed from the same atoms(they have the same molecular formula), but still differ. There are two kinds of isomers: Constitutional isomers and stereoisomers. Constitutional Isomers: Same molecular formula but different constitution(order of connectivity of atoms). An example would be C2H6O, which can come in the forms of methoxymethane(also called dimethyl ether for scrubs) and ethanol. Same molecular formula, but different constitution, and as a result differ in physical properties. Stereoisomers: Compounds with the same constitution but different spatial arrangements of the atoms. Oh boi. A basic example would be cis and trans stereoisomerism. As I can see, the two methyl groups in 1,2 dimethylcyclohexane are on the same side in the cis version of the molecule, while they are on opposite sides of the ring in the trans version. Why is this important? Think about what might cause these differences, and you will find out how this effects the properties of the molecule. Cis and Trans Molecules exhibit different physical, and maybe chemical properties. Recall that a π bond is formed by overlapping two p orbitals(look this shit up yourself if you dont understand, professor he gabe has more important shit to do). Rotation about the C-C double bond would effectively destroy the overlap between the p orbitals. Therefore, the C-C d-bond does not experience free rotation at room temperature. Cheeky chemists like to use cis-trans terminology to describe disubstituted alkenes, even when the two substituents(that are connected to the pi bond) are different from eachother. Watch out for this trap shit, and remember that cis-trans terminology applies to more, and for now you can disregard that meme terminology. Chirality Objects and their exact mirror images are said to be Superimposable. This means that they are the same thing basically, just viewed from a different angle(not really, but whatever)Disregard If an object looks exactly like it's mirror image, it is said to be Superimposable''.'' If an object does not look likes its mirror image, it is said to be nonsuperimposable. Hands are like this. Right and left hands are not identical, right hands will not fit into left hand gloves. Objects that are not superimposable on their mirror images arecalled chiral objects, from dead people lang meaning cheir(hand). All 3d objects can be classified as chiral or achiral. The most common source of molecule chirality is the presence of a carbon atom bearing 4 different groups. Consider 2-butanol, actually dont, my book uses this example but I dont want to write it out, use this convienent image instead. Anyways, the IUPAC recommended that a tetrahedral carbon bearing four different groups be called a chirality center, aka chiral center, stereocenter, stereogenic center, or asymmetric center. Since I'm dank and following a book that says this, I will be using chiral center as the name. When a compound if chiral, it will have one nonsuperimposable mirror image, call its enantiomer.(byzantine: 'opposite'). The compound and its mirror image are said to be a pair of enantiomers. Think of them as twins. I feel like going to bed, so goodnight class. Cahn-Ingold Prelog System We need to be able to identify each anantiomer individually. Cahn, Ingold, and Prelog made a system of doing this. This system involves assigning priorities to each of the four groups attached to the chiral center, based on atomic number. The atom with the highest atomic number is assigned the highest priority(1), while the atom with the lowest atomic number gets the lowest priorty(4). Take for example: Cl | H--'C'--O--R group | R group It would get assigned: Cl(1) | H(4)--'C'--O(2)--R group | R group(3) This number order goes the other way in the enantiomer. I aint making this shit again A counter clockwise sequence is designated S''', like what we have above. The enantiomer of this compound will be clockwise, so it will be designated '''R. S ''and ''R are used to describe the configuration of a chiral center. There are rules to this. # '' Prioritize all four groups connected to the chiral center # If necessary, rotate the molecule so that the fourth priority is on a dash # Determine whether the sequence 1-2-3 is clockwise or counterclockwise. Shit can get complicated H OH '\''' /// C3h7-C--'C'-C-C6H11-etc What do? Obviously oxygen gets priority 1, and Hydrogen priority 4, but whatabout garbon? Come with me and you will see that each carbon atom is connected to the chiral center and three other atoms. List 'em H OH \''' /// C3h7-C--'''C--C-C6H11-etc ^ ^ | | C,H,H C.H,H Since they are identical, you mve to the next atom in each direction and try again, and do this until you get a tie breaker, which ends up being the side with C3H7. When assigning priorities, double bonds are considered to be two separate single bonds. The following carbon atom is treated as if it is connected to two oxygen atoms. H | C-C=O ^ | O,O,H <---List for carbon After assigning values, rotate until the fourth priority is on the dash(facing away). Thats enoough for tonight class, had to rush it tonight. Also fuck formatting. Optical Activity Enantiomers exhibit identical physical properties. ®-Carvone has the same melting and boiling point as (S)-Carvone, for example. However, enantiomers do exhibit different behavior when explosed to plane polarized light. The reasoning doesnt matter, so lets just say dank memes are why. All you need to know is if a molecule is optically active, that means it has a chiral center. Miscellaneous info: For a compound with multiple chiral centers, a family of stereoisomers exists. Each stereosiomer will have at most one enantiomer, with the remaining members of the family being diastereomers. The number of stereoisomers of a compound can be no larger than 2^n, where n=the number of chiral centers. A meso compound contains multiple chiral centers but is nevertheless achiral, because it possesss reflectional symmetry. A family of stereoisomers containing a meso compound will have fewer than 2^n stereoisomers Speaking of symmetry, there are only two types: Reflectional symmetry and Rotational symmetry The presence or absence of an axis of symmetry(rotational symmetry) is irrelevant A compound that posses a plane of symmetry will be achiral A compound that lacks a plane of symmetry will most likely be chiral.